1. Field of the Invention
The present invention relates to an optical device such as binoculars, camera finders and head mounted displays (HMD) that are used to observe images through an optical system comprising lenses, mirrors and the like. More particularly, the present invention relates to an optical device having a dioptric correction device.
2. Description of the Prior Art
Human eyes normally perform appropriate focus adjustment depending on the distance between the object and the eye so that an observation object located at any point over the distance between close-range and infinity may be seen clearly. However, in the case of nearsightedness and farsightedness, this focus adjustment function does not work well, and consequently the range of distance over which the object may be clearly observed is limited.
For an optical device used to observe virtual images through optical lenses or mirrors, it is necessary to have a dioptric correction mechanism so that a sharp image is provided to the eye regardless of whether the observer has normal vision or is nearsighted or farsighted. A system in which the eye piece may be moved back and forth along its optical axis is the most commonly used dioptric correction device. For example, as publicly known in connection with binoculars, the position of the eye piece is adjusted by the observer himself while he is observing the image of the object so that the image can be seen clearly. Because the distance between the eye and the virtual image changes depending on where the eye piece is positioned, a virtual image position may be set at which the image can be observed with sharpness.
In an optical device in which image observation is made using both eyes, such as binoculars and binocular HMDs, it is necessary to perform dioptric correction independently for each eye because very often the observer's eyes suffer from differing degrees of nearsightedness or farsightedness. In this case, making adjustment separately for each eye, i.e., making dioptric correction for one eye first and then for the other eye, is the common practice. In addition, when dioptric correction is made for one eye, usually the other eye is closed while the adjustment is being made because the image seen by the other eye hinders the adjustment. Depending on the observer, however, dioptric correction may be made for both eyes with both eyes open.
If the object is within prescribed range of distance, observation of an object may be made clearly, because human eyes have the ability of focus adjustment. Therefore, when the observer himself performs dioptric correction so that clear image observation may be performed, a sharp image is obtained when a virtual image is at anywhere within said range. Therefore, even if the distance between the eye and the object is the same, the distance between the eye and the virtual image, however, is not maintained constant at all times but varies every time adjustment is made.
With regard to devices in which both eyes are used for observation, an example in which dioptric correction is made separately for the left and right eyes will be explained with reference to FIG. 12. In this case, when eye pieces 2 are moved for each of the eyes, one by one, to perform dioptric correction, both eyes 4 come to observe sharp images. However, because the degree of refraction is different from one eye to the other, and because human eyes, as described above, have the ability of focus adjustment, the distance between the eye and the virtual image is not necessarily identical for both eyes. For example, as shown in the drawing, there is a possibility that, as to right optical system R, the virtual image is located at position FR, which is the closest position to the eye within the range of distance over which the image is seen clearly, as shown by arrow AR, while as to left optical system L, the virtual image is located at a position within range AL different from said range AR which is the farthest position from the eye.
Thus, although each of the eyes can observe a sharp image independently, when both eyes are used for observation, the positions of the virtual images do not coincide. If image observation is performed while there is a difference between the eyes in terms of the distance between the eye and the image, the eyes become strained, and if this is continued over a long period of time, both the eyes and the brain become extremely fatigued.
In order to prevent such a problem, if dioptric correction is done for both eyes simultaneously while both eyes are open, when dioptric correction is performed initially for one eye, image observation may be made with a certain level of sharpness even if dioptric correction for the other eye is incomplete. As a result, image observation may be performed without fully completing dioptric correction for the other eye, and the adjustment operation is terminated without completely performing dioptric correction. Then, one eye observes a sharp image while the other eye observes a less sharp image, which causes strain and fatigue to the eyes.
In addition, when dioptric correction is performed by moving the eye piece back and forth from the position at which it is set at the time, there is no information as to which direction the eye piece should be moved. It thus becomes necessary to determine the direction in which it should be moved through trial and error. Moreover, dioptric correction using the conventional method described above is performed while the actual object of observation is being observed, and consequently when the observation object itself does not have a clear outline or sharp aspects such as straight lines, dioptric correction becomes quite difficult. In this case, the adjustment operation becomes rather time-consuming, and moreover complete dioptric correction may not even be possible.
As described above, in dioptric correction using a conventional method, because there is a certain range of distance in which the virtual image may be observed clearly, dioptric correction may result in a setting at which the observer experiences eye strain. Furthermore, depending on the observation object, adequate dioptric correction may not be performed and the purposes of dioptric correction may not be fully attained.
As publicly known, Japanese Laid Open Patent Hei 5-344450 shows an art in which images used for adjustment purposes are displayed on left and right image display units during the dioptric correction process in an eyeglasses-type display device having left and right image display units. The observer can perform dioptric correction using the sharp adjustment images, which makes dioptric correction easy.
However, even in this method, it cannot prevent the situation where the distance between the eye and the virtual image is not identical for both eyes. In addition, where dioptric discrepancy exists, information as to whether the eye piece is located off the position at which it would allow sharp observation in the direction away from the eye or in the direction toward the eye cannot be obtained. Therefore, in order to learn which direction the eye piece should be moved, the eye piece must be moved slightly back and forth from the position at which it is set at the time. Such a trial and error operation makes adjustment inconvenient.
Further, while this method is useful in an optical device such as HMD in which an image displayed on a display unit is observed, it cannot be applied in an optical device such as binoculars, in which an image other than the image displayed on the display unit is observed.
Moreover, in general, the shorter the distance between the eye and the object, the more strained the human eye becomes. If the virtual image is set at a position close to the eye in the dioptric correction process, while a sharp image can be observed, the eye becomes strained and fatigued.